1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding record sheets such as a paper laminated on a discharge tray or a document tray to an image outputting portion or an image reading portion while separating the record sheets one-by-one in an image forming apparatus such as a copier, a facsimile, a printer or the like, in details relates to an improvement in a sheet feeding apparatus capable of arbitrarily adjusting a separating force exerted between a record sheet to be transported and a sheet fed doubly with the record sheet.
2. Description of the Related Art
Conventionally, there is known a system for feeding record sheets laminated on a discharge tray while separating the record sheets one-by-one including a combination of a feeding roller and a separating roller applied with a reverse rotational torque. According to the system, a pickup roller is brought into contact with a topmost record sheet of the record sheets laminated on the discharge tray, the record sheet is drawn from the discharge tray by rotating the pick up roller and thereafter, a front end of the record sheet is entered s a nip portion formed by the feeding roller and the separating roller. Whereas the feeding roller is rotated in the same rotational direction as that of the pick up roller, namely, in a direction of feeding the record sheet further forwardly, the separating roller is applied with a rotational torque in a direction reverse to the direction of feeding the record sheet through a torque limiter and is brought into press contact with the feeding roller.
When the separating roller is brought into direct contact with the feeding roller (when the record sheet is not present at the nip portion) and when only one sheet of the record sheets is present at the nip portion, a rotational torque exceeding a limit value of the torque limiter acts on the separating roller by the feeding roller and the separating roller is driven to rotate by the feeding roller. Thereby, when only one sheet of the record sheets is entered the nip portion between the feeding roller and the separating roller by rotating the pickup roller, the record sheet is fed by the feeding roller and the separating roller is driven thereby to rotate.
On the other hand, when two or more sheets of the record sheets are fed into the nip portion between the feeding roller and the separating roller, the limit value of the torque limiter overcomes a friction force between the overlapped record sheets. Consequently, the separating roller is rotated in the direction reverse to the direction of feeding the record sheet to push back the record sheet on a lower side, with which the separating roller is brought into direct contact, toward the discharge tray. Thereby, whereas the topmost record sheet brought into contact with the feeding roller is fed by rotating the feeding roller, the record sheet entered the nip portion along with the topmost record sheet is brought back toward the discharge tray by rotating the separating roller. As a result, double feeding of the record sheets is prevented.
According to such a separating sheet feeding system, when a feeding force in a reverse direction exerted to a second record sheet by the separating roller is larger than a feeding force for dragging the second record sheet by a first record sheet to be transported, that is, a friction force acting between the first record sheet and the second record sheet, separating operation is acted between the first record sheet and the second record sheet and only the first record sheet is fed. Therefore, in order to firmly prevent the second record sheet from being transported with the first record sheet, it is necessary to precisely control the feeding force in the reverse direction acting on the second record sheet by the separating roller.
Since the feeding force in the reverse direction differs by a force of bringing the separating roller into press contact with the feeding roller, or a friction coefficient of the separating roller, according to a sheet feeding apparatus disclosed in Japanese Patent No. 3048685, attention is paid to a point that when a separating roller is rotated in accordance with a recording sheet passing a nip portion, a rotational number of the separating roller is changed with time. In the sheet feeding apparatus, when the rotational number becomes equal to or smaller than a predetermined value, it is determined that a friction coefficient of the separating roller is reduced and the force of bringing the separating roller into press contact with the feeding roller is enhanced.
However, according to the sheet feeding apparatus, the press contact force of the separating roller is changed based on the rotational number of the separating roller when a record sheet to be transported is fed and therefore, the press contact force is optimized when a successive record sheet is fed and the optimum press contact force cannot act on the record sheet when at least the rotational number of the separating roller is being measured. That is, with regard to the record sheet at which the rotational number of the separating roller is being measured, it is difficult to optimize the press contact force of the separating roller and therefore, there is a possibility that double feeding of the record sheets is brought about.
Meanwhile, according to a paper sheet separator disclosed in JP-A-2000-264489, temperature and humidity of a surrounding atmosphere of a nip portion are detected, a number of paper sheets entered the nip portion is detected, and by changing a rotational number of a feeding roller, a rotational torque in a reverse direction of the separating roller and a force of bringing the separating roller into press contact with the feeding roller based on the results of detection, the paper sheets are separated at high speed and firmly. Further, according to an sheet media separation device disclosed in JP-A-2000-044076, a separating roller is controlled to rotate in a direction reverse to a direction of feeding sheet media only when a plurality of sheets are entered a nip portion.
However, even when a plurality of record sheets is overlapped and entered the nip portion between the feeding roller and the separating roller, double feeding of the record sheets is not necessarily brought about but there is also a case in which even when, for example, first and second record sheets are overlapped and entered the nip portion, advancement of the second record sheet is hampered by the separating roller and only the first record sheet is normally fed. Therefore, it is not necessarily needed to reduce the force of bringing the separating roller into press contact with the feeding roller or to enhance the rotational torque in the reverse direction applied to the separating roller by entering a plurality of record sheets into the nip portion. Further, when the rotational torque in the reverse direction applied to the separating roller is unreasonably enhanced, an extra load more than necessary is applied to feeding the record sheet by the feeding roller and poses a problem that not only wear of the feeding roller is accelerated but also paper powders are liable to be adhered to the feeding roller and feeding of the record sheets becomes unstabilized.
Further, the separating force acting between the first record sheet and the second record sheet is varied also by a kind or a thickness of the record sheet set to the discharge tray and therefore, the separating force cannot pertinently be adjusted only by detecting a number of record sheets entered the nip portion, thereby a problem that double feeding of the record sheets is brought about, or the load of the feeding roller is increased more than necessary is posed.